Work on the interdependence of neurons and glial has focused on the regulation of CO2 fixation in neurons, astroglia and fibroblasts. The increase in CO2 fixation produced by increased extracellular potassium has been found to be specific for astroglial cells. The concentration range (2 mM to 50 mM K+) over which this effect on CO2 fixation was observed corresponds closely to the extracellular concentration of K+ surrounding neurons in the resting states. Ouabain, an inhibitor of Na+, K+ ATP'ase, has been found to have little or no effect on the stimulation of CO2 fixation by K+. Preliminary experiments with db c-AMP indicate that treatment of astroglial cells with this compound may increase their ability to fix CO2. These results suggest that release of both K+ and neurotransmitters from stimulated neurons may be important factors in regulating CO2 fixation in astroglial cells and therefore in maintaining an adequate concentration of citric acid cycle intermediates in both astroglial cells and in neurons.